110 MICROTINJE 



like the more normally shaped salient angles behind, each crest 

 consists of two intimately connected elements more or less 

 distinctly separated from each other by a little oblique furrow. 

 The two elements are, as in the other salient angles, the representa- 

 tives of two of the primitive tubercles, one belonging to the 

 median row, the other to one of the lateral (inner or outer) rows. 

 The homologies, as I understand them, of these parts are indicated 

 by the lettering to the figures. 



It will be seen from the figures that the salient angles and 

 re-entrant folds have a very different general appearance in these 

 young teeth from that which characterizes them in the adult 

 stages of wear. The salient angles or dentinal spaces are small 

 and narrow, the re-entrant folds or cement spaces are wide and 

 open, broadly U-shaped. These are characters common to the 

 teeth of all young voles, and they persist in the adult teeth of some 

 genera (e.g. Ellohius, Fig. 56). 



As the m-^ of Arvicola wears down its crown-pattern undergoes 

 a rapid transformation. The remnants of the crown tubercles 

 are quickly worn away, the ephemeral enamel islet representing 

 the worn-down median valley of the anterior loop being about the 

 last of these ephemeral features to disappear. The salient angles 

 and re-entrant folds acquire their adult form, the anterior sides 

 of the prisms becoming regularly concave, their posterior walls 

 convex. The two posterior outer and the three posterior inner 

 re-entrant folds persist as deep infolds which substantially close 

 the dentinal spaces ; but the third outer and fourth inner folds 

 gradually diminish in depth so that the fourth triangle becomes 

 broadly confluent with the fifth, and their common dentinal core 

 becoming broadly continuous with that of the anterior loop, 

 these two triangles are in the fully adult tooth part and parcel 

 of the anterior loop. The manner in which the third outer fold 

 is reduced is very instructive when compared with what happens 

 in certain species of Mimomys discussed below. Here and there 

 in adult teeth, long after all ordinary trace of youthful complica- 

 tion has disappeared, more or less well-developed vertical furrows 

 may be seen descending the posterior walls of the prisms or the 

 sides of the anterior loop (Fig. 59) ; traced upwards in younger 

 stages of wear these furrows are seen to meet the mouths of the 

 ephemeral valleys of the tubercular cap of the crown, and, as is 

 clearly shown in the genus Mimomys, such furrows are to be 

 interpreted as the functionless but persistent last vestiges of the 

 primitive tubercular cap. 



The late Pliocene and early Pleistocene genus Mimomys com- 

 prised, no doubt, the species which was the direct ancestor of the 

 late Pleistocene and recent species of Arvicola. To it, therefore, 

 we naturally look for further information as to the composition 

 of the primitive tubercular cap of the m^. Unfortunately, how- 

 ever, no unworn teeth of Mimomys have as yet been discovered ; 

 nevertheless, as Mimomys is a far more primitive genus than 



